Several questions:
1) Are you saying you need to sample 100 different inputs, or 100 samples of a single input; it's not clear from your post.
2) Precision and accuracy are not the same thing. What ACCURACY do you need? 8-bit? 24-bit?
3) Are you saying you need to store "a few weeks" worth of data? Unless you are sampling very slowly, you're going to need external memory.
In the meantime, look at the Microchip PIC24 series of micros. You can get them with up to 24 analog inputs (maybe more, I'm not sure). But if you need 100 analog inputs, you're going to need to add external multiplexers.